Journal Title (Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Page numbers: 2038-2045
Summary of Background and Research Design
Background: Ingestion of solutions with glucose (GLU) and fructose (FRC) leads to 20%-50% higher carbohydrate (CHO) oxidation rates compared with GLU alone. It is hypothesized that a high GLU oxidation rate may be beneficial to delay fatigue, increase endurance, and enhance performance, especially late in strenuous exercise when glycogen stores are depleted. Although most laboratory studies used solutions to deliver CHO, in practice, athletes often ingest CHO in the form of gels (semisolid). It is currently not known whether CHO ingested in the form of a gel is oxidized as effectively as CHO ingested as a drink.
Hypothesis:The authors hypothesized that exogenous CHO oxidation from CHO provided in semisolid (gel) or solution (drink) form during cycling is equally efficient and no difference in oxidation rate would occur.
Subjects: Well-trained male cyclists (N = 8) (age, 34 ± 7 yr; mass, 76 ± 9 kg; maximal oxygen consumption (VO2max), 61 ± 7 mL/kg/min) who trained >= 3 times/week for > 2 hours/session participated in the study. All athletes had been involved in running or cycling training for >= 2 years.
Experimental design:Randomized, cross-over
Treatments and protocol:One week before testing, an incremental cycle test to volitional exhaustion was performed to establish baseline values for maximal power output (Wmax) and VO2max. Three exercise trials were performed in random order (>= 5 days apart). Trials consisted of cycling at 59% ± 4% VO2max for 180 min at 50% Wmax while receiving 1 of 3 treatments: an isocaloric/isocarbohydrate gel (GEL), a 12.5% maltodextrin + fructose drink (DRINK), or plain water (WAT). The maltodextrins and fructose used in both the drink and the gel had high natural enrichments of 13C. After an initial bolus of 400mL DRINK or WAT ± GEL, a liquid and CHO dose was given at one half the loading dose every 15 minutes until cessation of exercise. Both CHO treatments delivered GLU plus FRC in a ratio of 2:1 at a rate of 1.8 g/min (108 g/hour). Fluid intake was matched between treatments at 867 mL/hour. Subjects were counseled to follow a CHO-rich diet (> 4g/kg body weight). Furthermore, 3 to 7 days before each experimental trial, subjects were instructed to perform an intense glycogen-depleting exercise session to reduce any endogenous 13C-enriched glycogen stores. Respiratory and blood samples were collected at rest and every 15 min until cessation of exercise. Breaths for 13C/12C ratios were collected at rest and during the last 60 sec of each 3-min exercise period.
Summary of research findings:
- Total fluid and CHO intake for each of the 3 exercise bouts was 2.6 L (867 mL/hour) and 325 g (108 g/hour), respectively
- Exogenous CHO oxidation from GEL and DRINK showed a similar time course, reaching peak exogenous CHO oxidation rates after 180 min of exercise
- Peak exogenous CHO oxidation rates were not significantly different (P = .40) between GEL and DRINK (1.44 ± 0.29 g/min vs 1.42 ± 0.23 g/min, respectively)
- Moreover, the average CHO oxidation rate over the final 120 min of exercise was not significantly different (P = .19) between GEL and DRINK (1.28 ± 0.26 g/min vs 1.24 ± 0.23 g/min, respectively)
- Oxidation efficiency was also not significantly different (P = .36) between GEL and DRINK (71% ± 15% vs 69% ± 13%, respectively)
- Plasma GLU levels were not significantly different between the CHO treatments (P > .05)
- The contribution of exogenous CHO to total energy expenditure was 39% for GEL and 38% for DRINK
- Endogenous CHO oxidation comprised 41%, 23%, and 25% for WAT, GEL, and DRINK, respectively
- As expected, endogenous CHO oxidation was significantly lower with both forms of CHO supplementation than with WAT during the last 120 min of exercise (P < .05)
- No increase in gastrointestinal distress was reported with either formulation of CHO compared with WAT
Interpretation of findings/Key practice applications:
There was no observed difference in CHO oxidation between CHO administered as GEL or as DRINK compared with WAT. This study found that supplementation with CHO GEL or DRINK is comparable and does not appear to be limited by gastric emptying or other factors that may slow CHO oxidation. Supplementation with CHO is recommended at ~1.5 g/min during endurance exercise, yet achieving this intake rate may be difficult with CHO solutions and the requisite large fluid intake. Furthermore, it is very likely that ingestion of CHO gels with a much lower fluid intake than used in this study would provide comparable results. Intake of CHO as a semi-solid GEL along with CHO beverages or water is an effective way to deliver high CHO intake rates, with limited GI tolerance issues, during prolonged endurance exercise. This study did not include a performance measurement, so it is not clear if both the gel and the drink supported performance equally well.